Regenerative drive systems act in a first manner to retard a vehicle such that motive energy is extracted from the vehicle's drive train and provides rotary (kinetic) energy to a pump/motor unit, which pumps a fluid from a reservoir to a higher pressure accumulator. Such drive systems also act to propel a vehicle by the reverse process, in which the stored (potential) energy of the fluid is released from the accumulator and drives the pump/motor unit to impart energy to the drive train. Two examples of such a system are described in U.S. Pat. No. 4,986,383 to Evans, issued on Jan. 22, 1991, and in U.S. Pat. No. 5,024,489 to Tanaka et al, issued on Jun. 18, 1991.
Regenerative drive systems have demonstrated the ability to cut fuel consumption in vehicles by 10-25%, although this is strongly dependent upon drive cycle and vehicle type (especially weight). There remains considerable interest in further improving such performance, either in the mechanical and hydraulic components themselves, or in control systems for the circuits constituted by these components.